Since the introduction of solid state devices (transistors, diodes, etc.), there has been a revolution in electronic products and devices. Almost all such items are manufactured using the latest in solid state electronic technology up to and including very large scale integrated circuits. Such devices are relatively inexpensive and contain up to hundreds of active devices on a very small chip.
The trend in design of almost all electronic circuits has been swept along with this revolution. For example, in the audio electronics industry, it is estimated that in excess of 99% of the electronic units are manufactured using solid state devices. Only a small fraction of the units are built using vacuum tube circuits.
Within the audio reproduction industry, however, there is a group of manufacturers and customers who are interested in the highest quality of audio reproduction that can be achieved. Within this group a paradox has arisen; there is a consensus of opinion that circuitry employing vacuum tubes reproduce sound with superior quality compared to circuitry employing solid state devices, even though conventional laboratory measurements which are used to compare performance yield data leading to the conclusion that solid state circuits perform better. Nonetheless, audiophiles with the financial means available often insist upon vacuum tube preamplifiers and amplifiers despite their bulk and higher cost.
Applicants have worked extensively in the field of high fidelity sound reproduction and in the elimination of spurious emanations in loudspeaker reproduced sound, as evidenced by U.S. Pat. No. 4,340,778 entitled "Acoustic Systems". Minimization of these spurious emanations has enabled identification of other disturbing factors, particularly those arising from different types of electronic system circuit topologies. It became evident from ongoing studies that numerous subtle influences affect an audio system that are inconstant in nature and derived from internal operative factors that have heretofore been accepted as necessary limitations on performance, given the need for reasonable system economy. The inconstant deviations of sound are perceptible to the human ear because of the great capabilities and subtlety of that organ, even though typical measuring instruments cannot quantify audible effects which are several orders of magnitude less than the dominant signal, and which vary dynamically in accordance with a complex multifrequency wave.
Applicants themselves first confronted this problem in the paradox between tube and solid state electronic devices when investigating aberrations in sound reproduction systems arising from specific devices. The investigations led to hypothesizing obscure phenomena in the devices and then performing tests to verify the hypotheses. Invariably, the effects yielded from the experiments could not be measured using conventional techniques, yet the effects could be reliably detected in sonic reproduction by knowledgeable and attentive listeners. Applicants subsequently devised techniques, described in U.S. patent application Ser. No. 467,434, filed Feb. 17, 1983 and entitled "Systems And Methods For Signal Compensation", for improving sound quality by counteracting the effects of nonlinearities in certain sound reproducing equipment. Thus, as disclosed in the previously filed application, immeasurable but nonetheless audible distortions arising from operation of a magnetic phonograph cartridge circuit can be eliminated by replication of hysteretic elements in a way which compensates for signal dependent nonlinearities.
It is recognized that the electronic device marketplace demands a level of total product performance (life, reliability, ruggedness, power drain, portability) that precludes the use of vacuum tube technology in all but a few cases. Thus, for instance, any development which would allow the use of conventional solid state circuits and devices while also attaining the level of quality of vacuum tube audio reproduction would be of significant economic value. Such an expedient could be applicable wherever circuit performance requirements entail the accurate reproduction of complex multifrequency waveforms. These areas include not only audio reproduction but also other applications such as video reproduction and thermal image reproduction.
The design of circuits for purposes of accurately reproducing signal waveforms generally deals with areas such as long and short term gain stability, signal-to-noise ratio, frequency and phase response and distortion of various types. In this pursuit, there are several conventional techniques which are used to reach the design goals set forth. Among these are the standard techniques utilizing feedback.
Local feedback can be used with any valve (transistor or tube) to minimize the nonlinear nature of its input-output transfer characteristic. This tends also to linearize the open loop nature of a combination of valves. To further improve the overall linearity of a combination of valves, the technique of overall negative feedback is employed to achieve the long and short term gain stability requirements. Feedforward is also employed, and it is to be noted that U.S. Pat. No. 4,379,994 suggests the inclusion of an error-generating device to correct for some characteristics of an amplifier, based upon a feedforward circuit and another amplifier. The errors involved, however, are the gross noise and distortion components for which typical compensating networks suffice as the error generator, and not the inherent internal nonlinearities of elements themselves.
Beyond the use of the conventional techniques, improvement in performance is generally achieved by improving the quality of the components--valves, capacitors, resistors, inductors, transformers, etc. The level in improvement which can be achieved in this fashion is generally limited by economic factors. Highest quality components often are disproportionately expensive relative to the incremental quality improvement that is achieved.